The present invention relates to nonwoven composite fabrics suitable for use for fine particle liquid filtration.
Nonwoven fabrics have been used for a variety of filtration and filtration-like applications. As an example, fine fiber webs such as meltblown fabrics and laminates thereof have commonly been used within air filtration media. Meltblown fabrics comprise a web of randomly inter-laid fine fibers, which provide a structure having excellent barrier properties. Generally, as the average fiber diameter decreases there is a corresponding decrease in the average pore size of the fabric. Thus, fabrics with finer fibers or smaller diameter fibers typically have increased barrier properties when compared to like webs of relatively larger fiber size. Therefore, due to fine fibers achievable in meltblown fiber webs and the excellent barrier properties resulting therefrom, meltblown fiber webs have been used in a variety of air filtration media such as, for example, in HEPA filters as discussed in U.S. Pat. No. 4,824,451, bag filters as discussed in U.S. Pat. No. 5,586,997, and filtering bacteria from fluids as discussed in U.S. Pat. No. 5,582,907 to Paul.
However, the needs of air filtration media often vary considerably from those of liquid filtration media. Notably, the particle size distribution within a liquid stream is typically much larger than particles associated with an air stream. In this regard, air filtration media are often expected to collect particles having a size less than about 5xcexc whereas with fine particle liquid filtration the particle size often varies between about 5xcexcto about 30xcexc. Multilayer filtration media suitable for air filtration, such as that described above, will often have an unacceptably short filter life when used for liquid filtration. While having an excellent filtration efficiency, the particles sizes associated with liquid filtration are typically of a size and distribution that the meltblown webs and/or laminates thereof quickly become fully saturated and/or create high pressure drops.
Additionally, meltblown fiber nonwoven webs can be relatively weak fabrics and often cannot, by themselves, withstand the conditions experienced by liquid filtration media. Thus, meltblown webs have been supported in multilayer structures to provide filter media or filter-like articles with improved strength and/or durability. In this regard, meltblown fiber nonwoven webs have been laminated with spunbond fiber nonwoven webs in order to provide a material with a combination of good strength and barrier properties. As examples thereof, spunbond/meltblown/spunbond media have been used in sterilization wraps and other like media such as, for example, those described in U.S. Pat. No. 5,464,688 to Timmons et al. and U.S. Pat. No. 4,041,203 to Brock et al. However, many nonwoven laminates are point bonded to form an integrated structure and, in this regard, the point bonds undesirably increase pressure drop without a corresponding increase in filter life and/or efficiency. Additional spunbond fabrics and/or laminates thereof utilized in filtration media are described in PCT Publication Nos. WO 96/13319 and WO 95/13856. Further, composite meltblown nonwoven fabrics, such as those described in U.S. Pat. No. 4,100,324 to Anderson et al., have also been used in liquid filtration applications wherein the composite nonwoven fabric is supported by a spunbond carrier sheet and a felt material.
However, there exists a need for filtration media suitable for use in liquid filtration that has good filtration efficiency and yet which also exhibit a suitable or even extended filtration life. Further, there exists a need for such materials which can provide the desired filtration efficiency and filter life and which are capable of servicing high volumes without creating high pressure drops. Still further, there exists a need for such materials that can be economically produced and which can withstand the pressures, handling and other conditions commonly associated with liquid filtration.
The aforesaid needs are fulfilled and the problems experienced by those skilled in the art overcome by the filtration media of the present invention comprising (a) a nonwoven composite material having a first and second side and comprising a matrix of thermoplastic microfibers having within said matrix at least about 50%, by weight, of a secondary material; (b) a first nonwoven web proximate the first side of the nonwoven composite material and comprising a substantially uniform nonwoven web of bonded fibers; and (c) a second nonwoven web proximate the second side of the nonwoven composite material such that the nonwoven composite material is positioned between the first and second nonwoven web. Desirably the nonwoven composite material and the first and second nonwoven webs form an integrated, autogenously bonded laminate. The nonwoven composite material desirably has a basis weight between about 30 g/m2 and about 300 g/m2 and further the secondary material of the nonwoven composite material desirably comprises a fibrous material such as, for example, pulp or polymeric staple fibers. The substantially uniform nonwoven material desirably comprises a nonwoven web having inter-fiber bonds throughout the web such as, for example, an autogenously bonded web of crimped polyethylene/polypropylene bicomponent fibers having a density between about 0.01 g/cm3 and about 0.2 g/cm3.
In a further aspect of the invention, liquids containing particulate matter can be filtered by providing the filter media of the present invention, supporting the filter media on a foraminous surface, and then drawing the liquid through the filter media, wherein particulate matter is collected in the filter media as the liquid passes therethrough. The liquid to be filtered desirably contains a substantial amount of particulate matter having a particle size of from about 5xcexc to about 25xcexc.